TeBG and transcortin were identified in serum from adult male baboons. TeBG values were higher than the TeBG concentration in adult men. The relative competitive abilities of three natural steroids, a synthetic androgen and two synthetic estrogens, for binding sites on TeBG occupied by radioactive DHT were compared. At a 10-fold molar excess, using both baboon and human TeBG, DHT and T were the most effective competitors; E2 was intermediate and DES a very poor one. Using gel filtration, the transcortin concentrations (mg/L) in two baboons were 16.8 and 27.4; the normal range for men is 23--45. The serum transcortin value of a third baboon, determined by equilibrium dialysis, was 0.41 micron/L (21.5 mg/L). Competition studies, using both techniques, showed that progesterone has a slightly higher affinity for baboon transcortin than cortisol and that a synthetic progestin does not bind to transcortin. Plasma binding capacity for synthetic steroids should be tested before they are used for studies on specific tissue receptors contaminated with serum.